In application Ser. No. 553,339, filed concurrently with this application in the name of the same inventor and commonly assigned, there is disclosed a novel corona discharge device comprising an elongated wire electrode and a shield partially surrounding the electrode, the shield being formed of a plurality of conducting segments insulated from each other and biased to a plurality of different potentials to create an asymmetrical electric field. The result of such a corona arrangement is to unbalance with respect to the wire electrode the density of ions across the ion delivery opening of the corona device.
This application is directed to the incorporation of such a device into a xerographic process as a means of conveniently controlling the transfer point under a transfer corona device. More specifically, in one stage of the xerographic process, charged toner is moved or transferred from the surface of a photoconductor to the surface of a copy sheet by bringing the front side of the sheet into contact with the toner covered surface of the photoconductor while concurrently applying a suitable charge of proper polarity to the other or back side of the sheet. When a given area on the back side of the sheet reaches a critical charge or transfer level, toner on the surface of the photoconductor opposite said given area is transferred or moved into contact with the adjacent front surface of the copy sheet. With a given transfer corona device, any given area of the back surface of the sheet will reach the critical transfer charge level at a distance under the corona device which is a function of various factors including humidity conditions, copy paper characteristics. The location or position of the given area under the corona device at which this critical charge transfer level is reached may be referred to as the transfer point. Since additional paper processing steps follow the transfer station in a xerographic process, it is important that the transfer point and the transfer of toner be fully completed well prior to further processing of the sheet. Since the transfer point may vary due to the above noted conditions, it is desirable to have a means for controlling this point as a function of such conditions during the xerographic process.